The current theories of the neurophysiological mechanisms of somatosensory perception emphasize the importance of cavity in primary afferent fibers of varying size and degree of myelination. In spite of studies performed over the last 75 years there is as yet no convincing analysis of the fiber size spectra and myelinated to unmyelinated ratios of afferent nerves. The impossibility of accurately measuring fiber in the 1-2 micron or less range using the light microscope is primarily responsible for this. Variations in fiber ratios have been proposed as the pathophysiological mechanism for such pathologic pain states for example as trigeminal neuralgia and post herpetic neuralgia. In addition while many physiological studies have attempted to locate the termination of primary afferent fibers related to pain perception at both the spinal cord and trigeminal levels no attempt at anatomic tracing has been carried out. We propose to utilize electron microscopic analysis of the fiber spectra of the spinal and trigeminal roots of various species in an attempt to establish their fiber size spectra and myelinated to unmyelinated fiber ratios of these animals. In addition we propose to trace the path and central termination of the fibers from the dental pulp, using electron microscopic radioautography. We believe that the variations in fiber spectra and ratios may represent an individual species recapitulation of the phylogenetic process of encephalization. Further, tracer method will provide anatomic evidence of the termination of primary afferent fibers related to pain perception which can be interpreted in the light of various physiological theories.